st/dri: use st->flush callback to flush the backbuffer

st/dri: use st->flush callback to flush the backbuffer

Previously the flush was done before the call to st->flush but
could lead to problems as FLUSH_VERTICES could push some work
that would change the backbuffer (or modify it).

With this commit, all the backbuffer flushing code is executed
right before the call to st_flush.

Closes: https://gitlab.freedesktop.org/drm/amd/issues/842
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=205049

Reviewed-by: Marek Olšák <marek.olsak@amd.com>